To make unified relocatable package easily, we may want to merge tarballs to single tarball like this:
zcat *.tar.gz | gzip -c > scylla-unified.tar.xz
But it's not possible with current relocatable package format, since there are multiple files conflicts, install.sh, SCYLLA-*-FILE, dist/, README.md, etc..
To support this, we need to archive everything in the directory when building relocatable package.
This is modifying relocatable package format, we need to provide a way to
detect the format version.
To do this, we added a new file ".relocatable_package_version" on the top of the
archive, and set version number "2" to the file.
Fixes#6315
536ab4ebe4